Fatigue Life of Low Carbon Steel under Programmed Loading Including Stress Lower than Fatigue Limit

Abstract

The manifold repeated rotating bending fatigue tests are carried out at two stress levels on three low carbon steels, which are rimmed, Si-killed and Al-killed steels having the different ageing abilities. These tests constitute a characteristic feature that the cumulative frequency of programmed loading cycles is mainly occupied by the understress lower than the fatigue limit.
The fatigue life under programmed loading is plotted against the equivalent stress, which is estimated on the basis of the modified Miner's law, both on log-log and semi-log diagrams. All experimental points of rimmed steel fall on the right side of the straight line extrapolated from the linear part of S-N curves on both diagrams. Those of Si-killed and Al-killed specimens seem to lie mostly around the above extrapolated straight line, but some experimental points in the longer life range tend to deviate from the straight line toward the shorter life direction on log-log diagram.
The cumulative cycle ratios based on Miner's law are mostly larger than one in rimmed steel, while in Si-killed and Al-killed steels the greater part of these values are less than one. These facts indicate that in the former the understress contributes to the increase of fatigue strength for the higher stress above the fatigue limit, while in the latter it gives fatigue damage to the specimen.
The results of surface structure observation and hardness measurement in the fatigued specimen suggest that the strain ageing gives a remarkable effect on fatigue strength. The fatigue test results mentioned above can be explained along this line.